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Disease guideIridovirosis (gill necrosis virus)

What is it?

Iridovirosis (gill necrosis virus) is a icosahedral DNA (deoxyribonucleic acid) virus affecting European flat oyster, Pacific oyster and Portuguese oyster.

Where and When Might it Occur?

A number of iridoviruses causing disease in oysters have been identified, but not all are associated with gill necrosis virus disease, and some affect oysters at different life stages.

Horizontal transmission occurs directly via the water column through the surface of the gills.

Little is known about the distribution of the organism responsible for this condition, but molluscan iridoviruses are generally considered to be distributed in oceans worldwide.

A protist, Thankatostrea polymorpha in the phylum Sarcomastigophora, has also been associated with this disease.

Outbreaks usually occur in spring and sometimes in summer.

Surviving oysters do not repair perforated gill structures and are potential carriers of the virus.


High mortality is often an indicator of the disease.

Gross pathological signs are:

  • yellow or green pustules on mantle or adductor muscle
  • yellow spots on gills and labial palps that spread as the disease progresses
  • spots that increase in size and develop brown centres as the tissue dies, leaving a hole in the gill structure.

Microscopic pathological signs are:

  • necrosis of gill or labial palp tissue
  • massive haemocytic cellular infiltration around lesions
  • basophilic, cytoplasmic inclusions found in most lesions.

SOURCE: Australian Government, Department of Agriculture, Fisheries and Forestry

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